Magnetometer core and process of making the same



1969- A. w. OBENSCHAIN 3,419,958

MAGNETOMETER CORE AND PROCESS OF MAKING THE SAME Filed Nov. 29, 1963ARTHUR W. OBENSCHAIN INVENTOR.

15 P BY ATTORNEY ,[ZMAXLQ AGENT United States Patent 6 Claims Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This application is a continuation-in-part of application, Ser. No.211,454, filed July 20, 1962, now abandoned.

This invention relates to a magnetometer core and to the process ofmaking the same including a novel annealing process. More particularly,the invention relates to a magnetometer core which in use exhibits alower noise characteristic than has heretofore been obtained, to theannealing process responsible therefor, and to a method of maintainingthe noise characteristic at the low value.

In magnetic cores heretofore employed with sensitive stationarymagnetometers it has been found that such stationary magnetic fieldmeasuring instruments of the fundamental frequency saturable core typeexhibit a noise associated with the magnetic core under a no-signalcondition which is often of higher intensity than the magnetic signal tobe sensed thereby. While the cause of this magnetic noise is notpositively known, it is thought to be molecular in nature as a result ofjumping of the magnetic domain within the core material forming smallsharp peaks of maxima similar to the well known Barkhausen effect.

In the heat treatment of magnetic core material heretofore employed ithas been the usual practice to:

(a) Furnace heat the material to 1250 plus or minus 10 C. and hold atthis temperature for two hours.

(b) Furnace cool the material to 1090 C. and hold at this temperaturefor one hour.

(c) Furnace cool the material again to a temperature of 400520 C. andhold at this temperature for twelve hours.

(d) Cool rapidly (250 C./hr.) to 200 C.

This old method possesses several disadvantages, for example, the hightemperature used (1250 plus or minus 10 C.) results in very soft coresthat are shock sensitive and easily damaged by rough handling. Anotherdisadvantage is the twelve hour hold at 400-520 C. which results in anoverly long annealing cycle. The greatest disadvantage of this oldmethod, however, is that the annealed cores consistently exhibit thehigh noise level mentioned above.

The magnetometer cores of the present invention possess all of theadvantages of the cores heretofore devised and none of the foregoingdisadvantages.

One of the objects of the present invention is to provide a magnetometercore having a magnetic noise characteristic not in excess of 50% of thecores annealed in accordance with the annealing process set forthhereinbefore.

Another object is to provide a core of magnetic material for use in amagnetometer having a noise level of about 50% or less than that ofcores heretofore produced and which possesses the desirablecharacteristic of ruggedness and resistance to change as the result of ashock applied thereto.

3,419,958 Patented Jan. 7, 1969 Other objects and advantages of thepresent invention will become apparent from the following detaileddescription of the invention when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 is a side elevation, partly in section, of a magnetometer coremade in accordance with the present invention; and

FIG. 2 is a cross-section taken on line 22 of FIG. 1.

Referring now to the drawings, the magnetometer core 10 of the preferredembodiment of this invention is seen to comprise a magnetic scroll 12composed of conventional high-permeability magnetic material mountedwithin a protective ceramic tube 14. The magnetic material employed maybe a material known in the trade as Permalloy having approximately 4%molybdenum, 79% nickel, 0.50% manganese and the remainder iron and asmall amount of impurities, or a material known as Supermalloycontaining 5% molybdenum, 79% nickel, 0.50% manganese and the remainderiron together with a small quantity of impurities. Another suitablematerial is that known as HyMu-80 manufactured by the Carpenter SteelCompany. The selected core material is first formed into a ribbon .0007inch thick having a length of 1% inches and a width of inch. The ribbonis then rolled to form scroll 12, 1% inches long, .064 inch in diameterand having approximately 1 /4 turns. Scroll 12 is disposed withinceramic tube 14 which is slightly longer than the length of the scrolland secured therein in the following manner. Tube 14 is provided nearits mid portion with an opening 16, through which a blunt tool isinserted to contact scroll 12 and to form a depression 18 therein. Careshould be taken here to avoid tearing or perforating the core material.Opening 16 and depression 18 are then filled with cement 20 to securethe scroll in the tube. The cement may, if desired, be trimmed down soas to be flush with the exterior surface of tube 14.

The forming operations which have been performed on the scroll, such asthe rolling operation and the forming of the depression 18, introducestresses within the material which stresses are ordinarily relieved by aheat treatment such as the process described above. The annealingprocess of the present invention, now to be de scribed, has been foundto be highly superior in that the resultant cores exhibit the lowernoise characteristic as discussed above.

A number of the core assemblies to be annealed are stacked horizontallyin a container suitable for transporting them through the heat treatmentand for storing them until they are to be assembled into themagnetometers. The assemblies, in their container, are placed in afurnace and annealed in a pure dry hydrogen atmosphere in accordancewith the following cycle:

(a) Heat at a rate of 400 C. to 500 C. per hour to 1,000 plus or minus10 C. and hold at this temperature for three hours.

(b) Turn off the furnace and cool the cores in the furnace, maintainingan average cooling rate of 30 to 40 C. per hour over the temperaturerange of 600 C. to 300 C.

(c) When the cores have cooled to 200 C. or less turn off the hydrogenand open the furnace.

Cores treated in accordance With this annealing process using varioushigh-permeability materials including 4-79 molybdenum Permalloy,Supermalloy, and HyMu- 80 have exhibited a low noise core characteristicof 1-2 gamma, in contradistinction to a minimum of 4 gamma noise levelof magnetometer cores heretofore devised. It was found, however, thatthe low noise characteristic so obtained was lost after a short time,apparently because the shocks incident to normal handling reintroducedstresses into core material. This additional problem was solved byvacuum impregnating the cores with hydraulic fluid, such as thatconventionally employed in power transmission systems. A suitable fluidis one which meets the requirements of Military Standard MIL-L-17111,although others may be used. The fluid is shown for illustrativepurposes in FIG. 2 at 22. In this process, a quantity of hydraulic fluidsuflicient to just cover the cores is poured into the containers of coreassemblies as soon as the last step of the heat treatment is completed.The container is placed into a vacuum chamber which is evacuated to asuitable low value and continuously pumped for a time suflicient otcause evacuation of the cores, about an hour having been found to besuitable. Air within the cores is drawn out and bubbled up through thefluid until the pressure within the cores is substantially inequilibrium with the zone above the fluid. The vacuum is then released,the resulting higher pressure on the surface of the fluid causing thefluid to be forced into the interior of the cores.

The fluid serves to support or cushion the core material and protect itfrom shock, thus preserving the low noise characteristic achieved in theheat treatment. To prevent loss of the fluid due to leakage orevaporation, the cores should be left submerged in the fluid until theyare to be assembled into the magnetometer. As an incident to assembly,the fluid may be sealed in, in any desired manner, but such is not partof the present process.

Whereas the invention has been described in detail with reference tothree particular types of magnetic core materials, it will be apparentto one skilled in the art, after understanding the invention, that othertypes of highpermeability core material may be employed withoutdeparting from the spirit and scope of the invention and it is intended,therefore, in the appended claims to cover all such types ofhigh-permeability material in the process of the present invention. Whatis claimed is: 1. The process of reducing and maintaining low the corenoise of high permeability magnetometer core material which haspreviously been formed into a thin ribbon, rolled into a scroll andsecured within a tube to form a core, which comprises the steps of:

heating the core in a furnace in a pure dry hydrogen atmosphere to atemperature of 1,000 plus or minus C.,

turning the heat off the furnace and cooling the core in the furnace ata rate of 30 to 40 C. per hour over the temperature range of 600 C. to300 C.,

when the core has cooled to 200 C., turning off the hydrogen andremoving the core from the furnace, and

vacuum impregnating the core with hydraulic fluid.

2. The process according to claim 1 which includes the additional stepof storing the annealed core in hydraulic fluid until it is assembled ina magnetometer.

3. The process of reducing and maintaining low the core noise of amagnetometer core comprising a thin ribbon of high permeability magneticmaterial, rolled into a scroll and secured within a tube, comprising thesteps of:

heating the core in a furnace in a pure dry hydrogen atmosphere to atemperature of 1000 C.,

turning off the furnace and slowly cooling the core in the furnace to200 C.,

removing the core from the furnace,

vacuum impregnating the core with hydraulic fluid, and

storing the core in hydraulic fluid until use.

4. The process of claim 3 wherein the 1000 C. temperature is maintainedfor three hours.

5. A process as set forth in claim 3, in which said vacuum impregnatingstep is performed by:

immersing the core in hydraulic fluid in a vacuum chamber,

drawing a vacuum on the surface of the fluid and maintaining said vacuumfor a time sufficient to cause evacuation of said core, and

releasing said vacuum whereby hydraulic fluid is caused to enter saidcore.

6. A method of making a magnetic core for a magnetometer, which coreexhibits a low noise characteristic, comprising the steps of:

rolling a flat strip of high-permeability magnetic material into ascroll,

inserting said scroll into a tube having an opening therein near the midportion thereof, inserting a blunt tool through said opening to make adepression in said scroll, and removing said tool,

placing a quantity of cement in said opening in engagement with saiddepression to secure the scroll in the tube and thus form said core,heating the core in a furnace in a pure dry hydrogen atmosphere to atemperature of 1000 C.,

turning off the furnace and slowly cooling the core in the furnace to200 C., and then removing the core from the furnace,

immersing the core in hydraulic fluid in a vacuum chamber,

drawing a vacuum on the surface of said hydraulic fluid to thereby drawa vacuum on the inside of said core, and

releasing said vacuum to thereby force hydraulic fluid into the core.

References Cited UNITED STATES PATENTS 1,270,969 7/1918 Parker 264-1012,102,683 12/1937 Dixon 148-122 2,656,290 10/1953 Beberich et al. 2641013,018,455 l/1962 Brandon et al. 336-213 X 3,127,559 3/1964 Legg et al336--213 X CHARLIE T. MOON, Primary Examiner.

C. E. HALL, Assistant Examiner.

US. Cl. X.R.

1. THE PROCESS OF REDUCING AND MAINTAINING LOW THE CORE NOISE OF HIGHPERMEABILITY MAGNETOMETER CORE MATERIAL WHICH HAS PREVIOUSLY BEEN FORMEDINTO A THIN RIBBON, ROLLED INTO A SCROLL AND SECURED WITHIN A TUBE TOFORM A CORE, WHICH COMPRISES THE STEPS OF: HEATING THE CORE IN A FURNACEIN A PURE DRY HYDROGEN ATOMOSPHERE TO A TEMPERATURE OF 1,000 PLUS ORMINUS 10*C., TURNING THE HEAT OFF THE FURNACE AND COOLING THE CORE INTHE FURNACE AT A RATE OF 30* TO 40*C. PER HOUR OVER THE TEMPERATURERANGE OF 600*C. TO 300*C., WHEN THE CORE HAS COOLED TO 200*C., TURNINGOFF THE HYDROGEN AND REMOVING THE CORE FROM THE FURNACE, AND VACUUMIMPREGNATING THE CORE WITH HYDRAULIC FLUID.